Linear aromatic condensation polyimides are being used increasingly as high performance film and coating materials by the electronic circuit industry. As cited by Senturia, Proc. of ACS Polym. Matls. Sci. and Eng., Vol. 55, 385, (1986), polyimides are being exploited for four primary applications in the area of microelectronics: (1) as fabrication aids such as photoresists, planarization layers, and ion implant masks; (2) as passivant overcoats and interlevel insulators; (3) as adhesives; and (4) as substrate components. Of utmost importance for the performance of a polyimide used for electronic applications is its electrical behavior. To be useful, particularly as a passivant or protective overcoat, the material must be an excellent insulator.
The dielectric constants of commercially available polyimides presently used as state-of-the-art materials for passivants and interlevel dielectrics range from approximately 3.2 to 4.0 (depending on frequency and moisture content). The lower limit of 3.2 is obtained on commercial polyimide film (Kapton.RTM. H film from E. I. DuPont de Nemours and Company) only after being fully desiccated. Unfortunately, as the film or coating absorbs moisture, the dielectric constant rises, making measurements and operation of electronic devices complicated.
The effectiveness of reducing chain-chain electronic interaction in lowering the dielectric constant of polymers was first described in the St. Clair et al patent application, "Process for Preparing Low Dielectric Polyimides", U.S. Ser. No. 073,542, filed July 15, 1987. St. Clair et al lowered the dielectric constant of polyimides by chemically altering the composition of the polymer backbone.
By the present invention, aromatic condensation polyimide films and coatings are produced which have dielectric constants that have been lowered by the incorporation of diamic acid additives. The use of low molecular weight additives has been described in the patent application of James C. Fletcher et al, "Polyimide Processing Additives", U.S. Ser. No. 084,064, filed Aug. 11, 1987. Those additives were used to decrease the melt viscosity of thermoplastic polyimides.
The incorporation of the diamic acid additives of the present invention into polyimides has provided materials that are better electrical insulators than state-of-the-art commercial polyimides. It is anticipated that these low dielectric polyimides will prove useful as film and coating materials for both industrial and aerospace applications where high electrical insulation, mechanical strength and thermal stability are required.
Accordingly, an object of the present invention is to provide a process for lowering the dielectric constant of an aromatic condensation polyimide.
Another object of the present invention is to provide a process for preparing aromatic condensation polyimide films and coatings having a dielectric constant in the range of 2.4 to 3.2.
Another object of the present invention is to provide a process for lowering the dielectric constant of an aromatic condensation polyimide by incorporation of a diamic acid.